Sex-linked inheritance
Genes on the X chromosome follow a different pattern in males (XY) and females (XX). A male has one X chromosome, so a recessive allele on it shows in him with no second copy to hide it.
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Analogy
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Example
Mother X^H X^h (carrier).Father X^H Y.Sons: 1 in 2 affected.Daughters: none affected,1 in 2 carriers.
In practice
Red-green colour blindness and haemophilia are X-linked recessive. They are more common in males.
Practice
Flashcards
- Why are X-linked recessive traits more common in males?Males have one X chromosome, so one recessive allele is enough to show the trait.
Interview questions
- Why are X-linked recessive disorders more common in males? Use a cross to support your answer.
- Males have one X
- Recessive allele shows without second copy
- Carrier mother passes to half of sons
- Daughters usually carriers
Questions people ask
Why can a father not pass an X-linked trait to his son?
A son receives the Y chromosome from his father and his X from his mother. So the father's X-linked alleles can never go to his sons. A father with an X-linked condition will pass his X to all his daughters, who will become carriers.
Can females have X-linked recessive disorders?
Yes, but it is rarer. A female needs two copies of the recessive allele, one from each parent, so her father must be affected and her mother must be an affected person or a carrier. Also, random inactivation of one X in each cell can occasionally cause mild symptoms in carriers.